This invention relates to control systems for electric motors and in particular to systems which are suitable for the accurate control of motors of which the speed/inertia ratio is low.
One feature of a control system for which the invention is intended is a comparatively low running speed of the driven motor. The significance of this feature is that if, as is usual, the speed of the motor is monitored, either directly by a tachogenerator or indirectly by means of a control track on tape driven by a capstan of which the motor is a part, the information rate obtainable is inevitably quite low. In practical terms this means that the speed of the motor or the tape is known, or measured, only at discrete times separated by intervals which may be quite long in comparison with the response of the motor. The effect of this is that the motor is capable of a significant change in speed between the intervals of sampling and accordingly the driving signal, which in ordinary control systems is determined by the measured speed signal, is inaccurate for most of the time. The system tends to instability, especially if the forward open loop gain of the system is high.
A second feature is that the inertia of the motor is correspondingly low. As previously indicated, this feature is not independent of the first and the invention is intended for use when the ratio of speed to inertia is low. Although a precise figure cannot readily be given, it will be apparent that such a ratio may often occur for a motor which is connected to drive a small capstan in a portable tape recorder. A specific embodiment of the invention concerns a capstan driven by a brushless DC motor. Such a motor has the advantages of reliable, quiet operation, independence of moisture and low friction. However, it has low inertia and, when used for driving a tape capstan, will be used at quite a low speed.
Whereas in a non-portable tape recorder a motor can be associated with a flywheel, which facilitates the obtaining of stability, such an expedient is unsuitable if low speed and low inertia is desirable, especially in a portable video tape recorder.
It is accordingly a general object of the invention to provide an improved control system for a motor.
It is another, more specific, object of the invention to provide a control system which is particularly suitable for motors of which the speed/inertia ratio is low.
Another, more specific object of the invention is to provide a capstan motor and control system suitable for use in a portable video tape recorder.